Our research program is aimed at determining the genetic and structural basis of antigenic polymorphism of the constant region (allotypes) and variable regions (idiotypes) of antibodies and at the genetic and cellular levels, the effect of regulation on the idiotypes and allotypes associated with specific anti-hapten antibodies. The possibility that allelic structural genes are responsible for the polymorphism of the H-chain variable regions is being investigated for anti-phosphorycholine antibodies. The expression of T15 Id associated with anti-PC antibodies and PC binding myeloma proteins of BALB/c and C3 Id with PC antibodies and binding myeloma proteins of C57BL/6 in backcross progeny and Ig congenic and recombinant inbred strains derived from these 2 strains suggest that these behave like allelic markers. IEF patterns of BALB/c IgG anti-inulin and anti-levan antibodies show that predictable restricted anti-inulin spectrotype patterns become more heterogeneous and complex in the presence of a regulator gene designated SR-1 contributed by C57BL/6. BALB/c (Igh-Ca) mice immunized with BL produce IgG anti-inulin antibodies of a single spectrotype. C57BL/6 (Igh-Cb) does not produce anti-inulin antibodies. (CxB)F1 produce IEF patterns that are considerably more heterogeneous and complex than BALB/c suggesting a regulatory gene SR-1 contributed by C57BL/6. SR-1 is not linked to Igh, MHC, or coat color genes.